The present invention pertains generally to plywood presses in which plywood panels are cured with heat and pressure.
During a pressing cycle the plywood panels in a press are typically subjected to pressures ranging between 100-225 PSI. Current practice in plywood press operation may entail the use of stepped pressures with the lower pressure values associated with the later stages of a pressing cycle. Reducing of pressure exerted on the plywood panels during the later stages of a cycle has been found advantageous in that compression losses (i.e., a reduction in veneer and hence panel thickness) may be reduced. Such compression losses are undesirable for the reason that the anticipated lost veneer and hence panel thickness must be pre-compensated for by peeling the veneer at a somewhat greater thickness resulting in reduced veneer production per block peeled. It should be noted that the length of press cycle time, veneer moisture content and glue type are all factors in veneer compression losses.
A discussion of compression losses is presented in an article entitled "Compression of Douglas Fir Veneer During Pressing" in Volume L11, Nos. 4 and 5 of a magazine titled THE TIMBERMAN. A further discussion is found in a report entitled "Compression Losses In Hot Pressed Plywood" published in August 1982 by the Forest Research Laboratory, School of Forestry, Oregon State University at Corvallis, Oregon.
Efforts to reduce compression losses, in addition to including multi-step plywood presses, have included mechanisms actuated by the thrust platen of a press which mechanisms limit compression to a percentage of the original thickness of the press load. Such mechanisms are disclosed in the above noted magazine article and are not in wide use in the plywood industry perhaps by reason of their complexity.
Plywood presses being used presently provide a multi-stage (multi-pressure) pressing cycle. Further efforts to reduce compression losses have entailed the installation of additional pressure controls to single or multi-stage presses all at considerable expense. The multi-stage presses, as presently known, provide only a partial remedy to over compression losses while incurring considerable capital expenditure and increased maintenance effort.
U.S. Pat. No. 4,367,115 disclosees an electronic control system for a circuit board press wherein pressures are altered during a press cycle. U.S. Pat. No. 4,151,718 discloses an electrical control system for a plywood press. The harsh conditions attendant plywood press operations render electrical systems impractical.